OB 105 

SB 
Copy 1 



Tlte golkf Tfai^it. 



THE 



SOLAR TRANSIT 



This Account of the Solar Compass, and the Meridian 

Attachment for Transit Instruments, was written 

for YOUNG & SONS, by Benj. H. Smith, 

of Denver, Colorado. 



PRICE 25 CENTS. 




PHILADELPHI 

YOUNG & SONS, No. 43 North Seventh Street, 



Copyrighted, 1887. 




YOUNG & SONS' 

No. 10, or MOUNTAIN TRANSIT, 

With B. H. Smith's Meridian Attachment. 

ST 

{Patented September 14, 1880.) 



Meridian Attachment 



FOR 



Transit Instruments. 



Although the Solar Compass has been in constant use for 
more than thirty years, and, with its modern improvements has 
become one of the most useful instruments known to the sur- 
veying profession ; yet, it is a remarkable fact, that its existence 
has been almost entirely ignored by the authors of all the 
modern text-books on surveying commonly used in schools 
and colleges. 

As a consequence, the young surveyor who soon finds the 
use of the solar apparatus attached to his transit indispensable 
in his practice, is obliged to resort to his own ingenuity to mas- 
ter the principles upon which the instrument is based, or depend 
upon the imperfect and often incorrect account to be found in 
the catalogue of the instrument maker he may happen to have 
on hand, 

To attempt to adjust and use any instrument without 
thoroughly understanding its principles, can only result in 
unreliable work, which fact has been notably demonstrated in 
the use, or rather abuse of the Solar Transit. 

It is the design of this paper to supply a clear and concise 
account of the instrument and its modifications, for the use of 
surveyors, and especially of those who may not be familiar with 
the astronomical problems involved in its construction, a brief 
explanation of which will first be given. 



The Diurnal Motion. 




For our purpose, the 
earth, so infinitely small in 
comparison with celestial 
magnitudes, may be sup- 
posed to be a fixed point C 
in space around which re- 
volves the celestial sphere 
ON HZ, Fig. i, in which Z 
represents the zenith and 
N the nadir of the ob- 
server ; PCP / the polar 
axis, about which the diur- 
nal motion is apparently 
performed ; O E H W the 
celestial horizon whose poles are Z and N, and AEQW the 
celestial equator whose poles are P and V '. All circles pass- 
ing through Z and N are called verticals, and those through 
P and P' are meridians or hour circles, the one passing also 
through Z being the meridian of the place, or simply the 

MERIDIAN. 

All the circles mentioned are great circles, that is, circles 
whose planes pass through C the center of the sphere. The 
circle BSD, described by the star at S in its diurnal motion, is 
a less circle. The distance from P or P' to any object on a 
meridian is called its polar distance, and the remainder of 
the quadrant measured from the equator is called its declina- 
tion. The distance from the zenith to any object on a vertical 
is its zenith distance, and the remainder of the quadrant to 
the horizon is called its altitude. Thus, P S is the polar dis- 
tance, L S the declination, Z S the zenith distance, and M S 
the altitude of the star S. O and H are the north and south 
points, and E and W the east and west points of the horizon. 
The direction of an object, whether terrestrial or celestial, with 
reference to the plane of the meridian, is called its azimuth or 
bearing, M or the angle P Z S is the azimuth of any object. 



S or M, in the vertical P S M reckoned from the north point. 
The angle Z P S, which the circle P S P' makes with the 
meridian, is called the hour angle of the star S. 

Careful observations of the motions of the stars, if continued 
for a sufficient period, will show that although their relative 
positions, and, therefore, polar distances and declinations, re- 
main unchanged, they all seem to revolve with a uniform mo- 
tion from east to west as though attached to the internal surface 
of a vast hollow sphere, having the observer in its centre and 
turning round the axis P C P', inclined to the horizon at an 
angle equal to the latitude of the piece. This apparent rotation 
of the heavens is called the diurnal motion. 

All bearings in land surveys being referred to the line H O, 
the correct determination of the direction of that line for any 
particular place is of utmost importance to the engineer and 
surveyor. The pole star whose polar distance is less than one 
degree and a half is a very convenient object for this purpose ; 
and, also, at the same time, for determining the arc P O or lati- 
tude of the place. 

The Engineer's Transit being an altitude and azimuth in- 
strument, can be used to take equal altitudes of the sun or of a 
star from which the meridian is readily determined, or the lati- 
tude and declination being known, the azimuth and hour angle 
may be computed from a single altitude of the sun or a star as 
follows : In the spherical triangle P Z S are given the three sides, 
P Z the complement of the latitude, P S the polar distance or 
complement of the declination, and Z S the zenith distance or 
complement of the altitude, whence P Z S (== O C M) the azi- 
muth, Z P S the hour angle, and Z S P the parallactic angle may 
be calculated. 

These methods all result in a sure determination of the 
meridian, but the time involved in taking and reducing the ob- 
servations is not always available to the engineer. Hence, the 
great value of a Meridian Attachment which will mechanically 
and instantly solve the problem above mentioned from a single 
observation of the sun, the apparent motion of which will be 
next considered. 




Fig. 2. 



If observations be taken 
at C, Fig. 2, during a whole 
year, it will be found that 
independently of the diur- 
nal motion which the sun 
has in common with the 
stars, it has also a motion 
in declination causing it to 
appear to describe annu- 
ally the great circle B T 
called the ecliptic, the 
plane of which forms an 
angle of about 23 28' with 
the celestial equator A O. 
The sun will appear to describe the circle B D at its greatest 
north declination on the 21st of June, and the circle E T at its 
greatest south declination on the 22d of December. Its declina- 
tion will be zero when crossing the equator on the 21st of March 
and 23d of September, respectively. 

The Meridian Attachment is simply an instrument made to 
imitate, on a small scale, the motion of the celestial vault as 
above described, consisting of a Solar Telescope revolving about 
its polar axis, w T hich corresponds with P C P / , in such a manner, 
that the line of collimation will follow the sun or any star in its 
apparent diurnal motion round the earth. Conversely when 
the transit is turned on its vertical axis to a position where the 
Solar Telescope when revolved on its axis will follow the sun 
or star, its axis must be in the line P C P', and, therefore, in the 
meridian of the place. This principle was first utilized, but for 
a different purpose, in the construction of the Universal Ring 
Dial, more than a century ago, and a description of that simple 
instrument, will best illustrate the subject for the reason, that 
all the many forms of solar attachments are constructed upon 
the same principles, and may be said to be mere modifications 
of the Ringr Dial. 



The Ring Dial. 




Fig. 3.— Ring Dial. H O, Horizon; P P, Polar Axis; A Q, Equator; 
A C Z = P C O, Latitude ; Ac b = A c \S, North Declination. 



The Ring Dial, Fig. 3, consisted of two rings of brass or 
other metal, which being turned at right angles with each other, 
corresponded with the equatorial and meridian circles AEOW 
and O Z H N, Fig. 1, and a plate turning on pivots at p and p' 
represented the polar axis P C P'. In an opening in this plate 
moved a brass block ^through a small aperture in which the 
sun's image was projected on a line engraved on the inner edge 
of the equatorial circle. The block d could be set to any re- 
quired declination A C B, by means of graduations along the 
opening in which it moved. The meridian circle was utilized 
as a latitude arc. The dial was suspended from a ring attached 
to the vernier / which was set to the latitude of the place, A Z 



8 

fc r 0) ; J hC ? iS ° f thC instrument ^en corresponded with 
the hne Z N and was revolved slowly thereon, until the suit 
.ma ? e crossed the equatorial line, the hour being ind cated bv 
graduates uoon the lower half of the equatorial cirde " 

With the instrument firmly fixed in this position the sun's 
.mage would follow the equatorial line, and, hence its axis of 
mottor t would correspond with^, which wouId J£j ™° f 
wuh^the polar axis PP, and, hence, with the direct.on of the 

The Ring Dial, if pr0 perly mounted, would therefore 
answer all the purposes of a Solar Compass in dete min n f t he 
mend.an as well as the time of day. ermining the 

Burt's Solar Compass. 




F.O. *-Ws Sol„Comp a „ H O Hortzon ; P P, Polar Axis; A Q , Equator . 
A C Z _ P C O, Lat.tude ; C A , _ A C E, South Declination. 



The first practical application of these principles to the art 
of surveying, was made by William A. Burt, of Michigan, in 
his invention of the Solar Compass, the prominent features of 
which are represented in Fig. 4. The bar f revolves in the 
plane of the equator A O about the polar axis jZ^', carrying the 
declination arc^ and bar de. The sun's image is brought to a 
focus at the intersection of lines engraved on a silver plate at e 
by means of a lens in the opposite end of the bar. To find the 
meridian, the latitude P C O is set off on the latitude arc /, and 
the declination A C E on the declination arc^. The Compass 
is then revolved about its vertical axis Z N, and the Solar ap- 
paratus about its polar axis until the image of the sun is brought 
accurately within the lines at e, when the axis//' must, neces- 
sarily, correspond with the plane of the meridian. 

This invention was originally designed, and was admirably 
adapted for use in connection with the open sight compass for 
work on the public land surveys, but when the Engineer's 
Transit came into more general use, and a higher order of 
land surveying was demanded, various attempts were made to 
attach the Burt apparatus to the Transit, but never with satis- 
factory results. It has been mounted over the needle box, 
under the main plate, on the end of the axis, on top, and even 
on the object end of the Telescope, but in every case at the 
expense of the usefulness of the Transit. 

The Meridian Attachment. 

With a view of meeting these objections, the writer, a few 
years ago, designed a form of Meridian Attachment, especially 
for use in connection with the Engineer's Transit, which has 
since been manufactured by Messrs. Young & Sons, to whom 
belongs the credit due to skillful workmanship and good judg- 
ment in the arrangement of the details of construction. After 
six years' trial in the field, it is found that the following advan- 
tages over the old form have been secured : 

(a) Compactness of form, especially adapting it for attach- 
ment to the Engineer's Transit. The instrument being com- 
plete in itself, the Transit Telescope and vertical arc are not 



io 



required to do double duty. The view of the needle box and 
verniers is unobstructed. 

(b) The sun being observed through a telescope, its image 
can be brought sharp and clear between the equatorial wires 
with greater exactness than can be attained by the old plan of 
focussing upon a silver plate ; and, hence, the meridional result 
is more accurate. 

(c) The polar axis, which from the preceding remarks will 
be recognized as the vital part of a Solar Attachment, is longer 
than in any other form. 

(d) The sun's image can be clearly defined in hazy weather 
when the old forms cannot be used at all. 




Fig. 5. — Smith's Meridian Attachment. H O, Horizon; P P, Polar Axis: A Q, 
Equator ■ ACZ = PCO, Latitude ; ac i> ^ A C B, North Declination 



11 

The Meridian Attachment is represented in Fig. 5 , C is the 
Solar Telescope revolving in collars r and r' t whose line of 
collimation and axis of revolution coincide with the polar axis 
P P / '. The declination arc d is fixed to the side of the Tele- 
scope, the vernier being attached to the arm e which turns on 
-its axis a reflector at c in front of the object-glass of the Tele- 
scope. The collars in which the Telescope revolves are firmly 
attached to the latitude arc /, having a horizontal axis, the whole 
being mounted on the frame yy 7 which is attached to the stan- 
dards of the Transit. Tangent screws / and t f give slow mo- 
tions to the declination arm and latitude arc. 

The arm e is so adjusted that the declination vernier reads 
zero when the plane of the reflector makes an angle of 45 with 
the axis of the Telescope, in which position the line of collima- 
tion is reflected at right angles, and is caused to coincide with 
the line a c parallel with A C. Hence, if the Telescope be 
revolved on its polar axis, the line of collimation will describe 
the celestial equator AEO W, Fig. 1. In like manner, by set- 
ting off on the declination arc any given declination north or 
south, as A C B or A C E, the image of any celestial object 
traversing the circles B D or E T, Fig. 2, may be kept in the 
center of the field of view from rising to setting, by simply 
revolving the Telescope. The hour arc is attached to the Tele- 
scope at //, revolving at right angles with the polar axis, and, 
hence, in the plane of the equator. 

The appearance of the sun in the field 
of view is represented in Fig. 6. The 
three equatorial wires a b correspond 
with the line of the celestial equator and 
circles parallel therewith, while the hour 
wire c d corresponds with the hour circle 
or meridian towards which it may be 
directed, 

The meridian is found in precisely the 
same. manner as with the Ring Dial and Burt Solar Compass. 
Having set off the latitude and declination, and the hour circle 
to the approximate time, the sun can generally be brought into 




12 

the field of view by simply revolving the Transit on its vertical 
axis. The Transit then being clamped, the sun may be brought 
accurately between the equatorial wires with the tangent screws, 
at which time the Solar Telescope and also the Transit Tele- 
scope parallel to it, will be in the plane of the meridian. 

The same letters and lines have been used in the foregoing 
figures for purposes of comparison, and to show the reader how 
the three instruments described are based upon the same prin= 
ciples. 

Latitude. 

It will be readily understood from the above explanations, 
that the Meridian Attachment in common with all forms of solar 
devices for determining the meridian, depends for accuracy of 
results upon the indispensable condition that the polar axis of 
the instrument must coincide with the line P P 7 , Fig. 2, at some 
time during a revolution of the Transit on its vertical axis. 
Hence, the most important requirement is a correct determina- 
tion of the angle P C O or the latitude of the place of observa- 
tion. Whether ascertained from accurate maps or charts, or 
from direct observations by some of the well-known methods, 
the latitude should be known within one minute to ensure a 
correct meridional result. 

Declination and Refraction. 

The apparent declinations of the sun at Greenwich mean 
noon, and the hourly differences may be found in the Nautical 
Almanac. To calculate the declinations, it requires an approxi- 
mate knowledge of the longitude of the place, which can be 
determined from any good map with sufficient accuracy. 

The effect of refraction is to apparently increase the altitude 
of celestial objects. Tn Fig. 2, if B D and E T represent two 
diurnal circles described by the sun, one with north and the other 
with south declination, the dotted lines r and r' will represent 
the apparent path of the sun as affected by refraction. It will 
be seen that the effect on the declination will be to apparently 
increase it when north, and decrease it when south. The cor- 



13 

rection is, therefore, made by adding the correction for refrac- 
tion to north, and subtracting it from south declinations. 

The tables of refractions being calculated for verticals, can- 
not be applied to declinations, which are measured on meri- 
dians, without special computations. Hence, the necessity of 
the table of refractions in declinations, prepared by Hon. 
Cortez Fessenden, and published for use with the Solar Transit 
by Young & Sons. The table is calculated for all hours of the 
day, and for any latitude from 30 to 55 . 

The method of calculating the declinations for a day's work 
can be best illustrated by an example : 

Time, November 1, 1886. Station, Denver, Colorado. 

Latitude 39 45' N. Longitude 105 oo / W. 

From the Nautical Almanac, 

Declination, S. 14 30 / 19. 4". Difference for 1 hour — 
48. o", the — sign indicating that south declination is increasing. 

From table of refractions in declination for nearest latitude 
40 and declination — 15 , the corrections are for noon, V 2i 7/ , 
1 hour i / 25", 2 hours i / 35", 3 hours 2 / 01", 4 hours 3' 18". 

Reducing the longitude 105 to time by dividing by 15 (15 
of arc = 1 hour) gives 7 hours ; and, therefore, Greenwich noon 
corresponds with 5 A. M., at Denver. The declination being 
south, the corrections for refraction are subtracted, hence, the 
following results : 



Time. 


Declination. 


Refraction. 


v^orrecicu 
Declination. 


5 A.M. 


14° 


3 Q/ 


19-4" 






+ 3X48" = 




2 


24 






8 A. M. 


H 


32 


434 


- y i8« = 


14 2 9 / 25.4 


+ 48" 












9 A. M. 


14 


33 


314 


— 201 = 


14 31 3O.4 


10 A. M. 


14 


34 


19.4 


— 1 35 = 


H 3 2 444 


11 A.M. 


H 


35 


07.4 


— 1 25 = 


14 33 424 


Noon, 


14 


35 


55-4 


— 1 21 = 


14 34 344 


1 P.M. 


14 


36 


43-4 


— 1 25 = 


14 35 184 


2 P.M. 


H 


37 


314 


— 1 35 = 


H 35 564 


3 P. M. 


14 


38 


19.4 


— 2 01 — 


14 36 18.4 


4 P.M. 


14 


39 


07.4 


- 3 '3 


14 35 494 



14 

As the declination arc is only graduated to minutes, the 

results may be transferred to the field book for reference, as 

follows : 

November I. 1886. 

Noon, 1 4 34' 344 // - 



8 A. M., 


14° 29/ 


1 P.M., 


H° 35 


9 A.M. 


14 3i 


2 P. M., 


14 36 


10 A. M. 


H 33 


3 P. M., 


14 36 


11 A.M., 


H 34 

Ad 


4 P. M., 

iustments. 


H 36 



From the foregoing synopsis, it will be apparent that the fol- 
lowing conditions must be established in the construction of a 
theoretically perfect Meridian Attachment : 

(a) The Transit to which it is attached should be in perfect 
adjustment. 

(b) When the optical axis of the Solar Telescope is hori- 
zontal, the latitude vernier should read zero. 

(c) The axis of the reflector should be at right angles with 
the optical axis of the Telescope, and when its plane is at 45 
therewith, the declination vernier should read zero. 

(d) The axis of the latitude arc should be horizontal and at 
right angles with the optical axis of the Transit and Solar Tele- 
scopes. 

(e) The equatorial wires must coincide with the line of the 
celestial equator, and the hour wire be at right angles there- 
with. 

It is supposed that the reader is familiar with all the adjust- 
ments of the Transit, but it may be worth while to remind him 
that good solar work cannot be effected unless his Transit is a 
first-class one, and in perfect adjustment. 

The adjustments of the Meridian Attachment are as follows : 
The Latitude Vernier. Set the latitude arc at zero, clamp 
it, and place the striding level upon the Telescope. Bring the 
bubble to the centre by turning the tangent screw /. Then 
reverse the level, and if the bubble settles in the same position 
as before, we may conclude that the axis is horizontal ; but, if 



15 

the bubble moves from its former position, turn the screw so as 
to move the bubble over half this distance, the other half to be 
ascribed to error in the level itself. If, when the level is re- 
versed, the bubble occupies a similar position in the opposite 
direction, the adjustment is complete. The vernier will now 
indicate the index error, which may be corrected by shifting the 
vernier by means of the adjusting screws for that purpose. 

The Decimation Arc. Having set off the latitude, take an 
observation of the sun on the meridian, and. bring its image 
accurately between the equatorial wires by means of the tangent 
screw t' . The difference between the observed and calculated 
declinations corrected for refraction will be the index error, 
which may be corrected by loosening the three small screws on 
top of the arc, and moving the arc to the correct reading. 

The Plane of the Latitude Arc. The axis of the latitude arc 
and that of the reflector should be placed by the maker at right 
angles with the optical axis of the Solar Telescope, and are not 
liable to derangement. The vertical planes of the latitude arc 
and the Solar and Transit Telescopes should also be made 
parallel ; but as this condition is sometimes disturbed in detach- 
ing and attaching the apparatus to the standards, the following 
is the adjustment : 

Having completed the adjustments above described, take a 
solar observation at say, 9 A. M., and note the error east or 
west of the meridian as indicated by the Transit Telescope 
directed south. Bring Transit Telescope to the meridian with 
the tangent screws. This will cause the sun's image to leave 
the equatorial wires diagonally. Then by means of the small 
butting screws in the plate// 7 , move the south end of the plate 
east, if the error was east, or west, if it was west, until the sun 
is accurately between the wires. A solar observation at 3 P. M. 
will verify the adjustment; but, if the morning and afternoon 
observations cannot be made to agree, then a portion of the 
error must be ascribed to the plane of the reflector not being 
truly at right angles with the line of collimation. The adjust- 
ment of the reflector should be perfected by the maker, and is 
not liable to get out of order. The surveyor is recom- 



16 



MENDED, THEREFORE, IN SUCH A CASE TO ALLOW FOR THE 
MERIDIAN ERROR IF SMALL, BUT IF LARGE, TO RETURN THE 
INSTRUMENT TO THE MANUFACTURER FOR RE-ADJUSTMENT. 

The J'lquatorial and Hour Wires. If the sun in traversing 
the field of view should appear to depart from the equatorial 
wires, the correction can be made by loosening the screws and 
rotating the diaphram carrying the cross wires, until the sun 
appears to follow the equatorial wires accurately. 

All these adjustments are made by the manufacturer, and 
are not liable with careful usage, to become deranged. They 
should, however, always be verified before beginning any im- 
portant work. 



Young 6° Sons' Catalogue of Engineering, Mining and 
Surveying Instruments, mailed upon application. 



~? RY 0F CONGRESS 



I 

'III!: 



003 608 548 J 



